A Raman scattering study of the phase transitions in SrTiO₃ and in the mixed system (Sr_1-xCa_x)TiO₃ at ambient pressure from T = 300 K down to 8 K

Abstract

The room-temperature Raman spectra of Sr_1-xCa_xTiO₃ (SCT) for x = 0.06 (SCT06), 0.12 (SCT12) and 0.30 (SCT30) are shown to contain first-order Raman lines corresponding to E_g and B_1g modes, which appear below 105 K in pure SrTiO₃. The E_g line shows slight asymmetry, indicating the orthorhombic structure of SCT for x≥0.06. The low-temperature Raman spectra of SCT06 and SCT12 contain Raman-active modes of symmetry B_2g and E_g, predicted by group theory but not observed in SrTiO₃ or SCT0.7 (x = 0.007) by previous workers. The polar hard modes TO2 (175 cm^-1) and TO₄ (550 cm^-1) are present even at room temperature for SCT06 and SCT12 due to Ca²⁺-centred ferroelectric microregions. The intensity of these modes starts increasing below 55 and 80 K for SCT06 and SCT12 respectively, which are nearly 2 T_c, where T_c is the temperature of the smeared dielectric peak. The presence of such an extended precursor region is akin to relaxor behaviour. For SCT30, two new lines at 79 and 128 cm^-1 appear below the paraelectric to antiferroelectric phase transition temperature of 230 K. It is shown that Raman lines characteristic of both ferroelectric and antiferroelectric phases are present in SCT12 below 100 K, confirming the frustration model for the highly smeared dielectric response for this composition.